Rick Perry, one of the leading Republican contenders for the 2012 White House race, is under fire after organising a huge Christian evangelical rally at a football stadium in Houston, Texas.

Although he has not yet formally declared his intention to stand, Perry is in second place in some polls behind front-runner Mitt Romney. But critics say he is risking his reputation on Saturday's planned day of prayer and fasting that he has declared in response to the economic and moral crisis facing the country.

Perry, the governor of Texas, has been accused of mixing politics and religion, and faces questions over the organisation financing the rally – which has been described as a "hate" group by discrimination monitors. Atheists filed an unsuccessful lawsuit to have the rally stopped and Perry's involvement declared unconstitutional.

Perry, in a message inviting Christians to the rally, said: "Right now, America is in crisis. We have been besieged by financial debt, terrorism, and a multitude of natural disasters. As a nation, we must come together and call upon Jesus to guide us through unprecedented struggles, and thank him for the blessings of freedom we so richly enjoy."

He added: "Some problems are beyond our power to solve, and according to the Book of Joel, chapter 2, this historic hour demands a historic response. Therefore, on 6 August, thousands will gather to pray for a historic breakthrough for our country and a renewed sense of moral purpose."

He called on Christians to join him at the 71,000-seat Reliant football stadium for the seven-hour rally, "with praying people asking God's forgiveness, wisdom and provision for our state and nation. There is hope for America. It lies in heaven, and we will find it on our knees."

He is to announce either this month or next whether he will seek the Republican nomination for the presidency. He has already put a campaign team in place and begun sounding out fundraisers.

If he stands, he would likely become the frontrunner, provided the rally does not rebound on him.

He has won the governorship of Texas three times in a row and the state has a strong record on employment, accounting for about a third of new US jobs, providing him with a good platform in a country where unemployment is the number one issue.

Although the rally, called "The Response", has created a lot of dissent, it could help Perry win over Christian evangelicals. They make up about half of the Republican grassroots support, according to polls, and an even higher proportion in key caucus and primary states such as Iowa and South Carolina, two of the early states that can be decisive in the nomination battle.

But the New York Times reported this week that only about 8,000 people had signed up for the event. Perry invited all the country's governors but only one, Sam Brownback, of Kansas, is to attend. Sixty-thousand empty seats might be seen as a humiliating rebuff.

Eric Bearse, a rally spokesman, told CNN the attendance was not an issue.

"Not concerned whatsoever. We think it will be a powerful event whether it is 8,000 or 50,000. The only people concerned about numbers are press," he said.

The American Family Association, formerly known as the National Federation for Decency, is financing the rally. The organisation, based in Tupelo, Mississippi, runs about 200 radio stations. The Southern Poverty Law Centre, which tracks discrimination, described it as a hate group.

Critics of the rally, including other Christian groups, were scheduled to hold a counter-rally on Friday. Ministers from some churches signed a letter critical of Perry for holding a rally that they say is only for Christians and excludes Muslims and other faiths.

The Houston Clergy Council, in a letter to Perry, said: "The American Family Association and its leadership have a long track record of anti-gay speech and have actively worked to discriminate against the gay, lesbian, bisexual, and transgender community."

It added: "We ask that Rick Perry leave the ministry to us and refocus his energy on the work of governing our state."

But Perry is to follow up the rally by speaking at Virginia's Liberty Christian Academy, one of the powerhouses of the evangelical movement, on 14 September.